In the bottom of your fuel tank is this strainer.
It screws into the tank from under your car. As it collects more and more dirt, sludge, algee etc....
less fuel can go through it to the engine.
There will be about a 6" piece of hose connected from it to the steel fuel line going to the engine.
This strainer can be removed and cleaned. First it is wise to drain the tank or siphon it before removing .
Good info in this thread. Don`t know the final out come, they come and get help
but don`t let us know if it fixed it or???
http://www.peachparts.com/shopforum/diesel-discussion/360842-power-loss-mystery-not-filters.html
https://www.google.com/search?q=mercedes+w123+fuel+tank+strainer&biw=1012&bih=482&source=lnms&tbm=isch&sa=X&ved=0ahUKEwihmOPc3L_OAhVX5WMKHQPQAYkQ_AUIBygC#tbm=isch&q=site+peachparts%2C+w123+fuel+tank+strainer
Each picture is from a different thread from this forum. click on them and will get you to that thread.
Another problem could be the vent for the tank becoming plugged.
Look behind the L/R wheel, and you will se a small bell shaped piece,
this is the end of the vent.